The next two functions allow for accessing all items in the database. This
access is not key sequential, but it is guaranteed to visit every
key in the database once. The order has to do with the hash values.
gdbm_firstkey starts the visit of all keys in the database.
gdbm_nextkey finds and reads the next entry in the hash structure for
dbf.

key = gdbm_firstkey(dbf);
nextkey = gdbm_nextkey(dbf, key);

The parameters are:

GDBM_FILE dbf

The pointer returned by gdbm_open.

datum key

datum nextkey

The key data.

The return values are both datum. If key.dptr or nextkey.dptr is NULL,
there is no first key or next key. Again notice that dptr points to
data allocated by malloc and gdbm will not free it for you.

These functions were intended to visit the database in read-only algorithms,
for instance, to validate the database or similar operations.

File visiting is based on a hash table. gdbm_delete
re-arranges the hash table to make sure that any collisions in the table do not
leave some item un-findable. The original key order is NOT guaranteed to
remain unchanged in ALL instances. It is possible that some key will not be
visited if a loop like the following is executed: